The usage of polyurethane foams has greatly increased in the last ten years due to innovations and improvements leading to enhanced physical properties. These enhanced physical properties include flame retardation characteristics and the production of foams which do not drip when subjected to high heat. These polyurethane foams are based on the reaction of a polyisocyanate, such as toluene diisocyanate, and either a polyether or polyester polyol. For convenience herein, at times "polyether polyols" will simply be referred to by the term "polyol," and "polyester polyols" will be referred to by the term "resin" or polyester resin.
In the art it is generally recognized that polyether polyols conventionally have a greater functionality than polyester resins, i.e., polyether polyols are predominantly trifunctional and polyester resins are predominantly difunctional. Similarly, it is generally accepted that polyurethane foams based on polyester resins, because of the ester linkages, tend to hydrolyze and, accordingly, are unacceptable for certain usages, for example where the foams are subjected to moist or humid environments. It is also generally accepted that polyether-based polyurethane foams have poor flame laminating characteristics. Moreover, it is known that polyether polyols, because of their lower viscosity, are more difficult to pour or lay down on a foaming surface without splashing during pour. This splashing has a tendency to trap air in the foam-forming mixture as it is being poured, forming air bubbles in the foaming mixture as it commences rising. These air bubbles cause holes in the final foam product which is detrimental to many applications, particularly those involving thin sections of foam.
It is recognized in the art that the softness of a foam, including a polyether foam, can be increased by reducing the amount of polyisocyanate used in the foam-forming reaction mixture. It is also recognized, however, that below a certain minimum level of diisocyanate, the foam characteristics are damaged, including to the extent that the foams do not have good structural integrity. It also has been recognized that the softness of foams can be increased by utilizing select blowing agents, such as the chloro-fluoro carbons. Chloro-fluoro carbons, however, are now substantially unacceptable as foaming agents because of their effect on the ozone layer. It is also possible to provide polyether foams having good flame laminability characteristics. However, all such flame laminable polyether-based polyurethane foams must have additives present which increases the cost of the foams and also leads to greater difficulty in the production of the foams.
Accordingly, in spite of all of the recent innovations in the preparation of polyurethane foams there is still a need for polyether foams wherein the splash characteristics are reduced or eliminated which, in turn, will provide a hole-free polyether foam. There is also a need for polyether foams having increased softness without affecting the physical characteristics of the foam and without use of blowing agents which have a damaging effect on the environment. There is also a need for polyether foams which are flame laminable and which pass stringent flammability tests without having additives which increase the foam costs and complexity of the foam-producing operation. Finally, there is a need for polyester foams having improved hydrolytic stability. The present invention provides foams having all of these desirable characteristics.